Many cells have the ability to sense the direction of external chemical signals and respond by polarizing and migrating towards chemoattractants. This phenomenon, called chemotaxis, has been shown to play an important role in embryogenesis [1], wound healing [2, 3], immune response [4] and cancer metastasis [5, 6]. In addition, cell migration and trafficking are closely associated with relevant physiological problems and diseases such as autoimmune diseases and cancers, and therefore has high clinical relevance. The ability to observe a single cell's response to a chemotactic environment is necessary in order to develop quantitative models to describe and predict chemotactic behaviors.
Conventionally, cell chemotaxis is measured in vitro by Boyden chamber or transwell assays and other free-diffusion based cell migration assays, such as under-agarose assay, micropipette-based assay, Dunn chamber, and Zigmond chamber [7-11]. Although widely used, these assays suffer from poorly controlled chemoattractant gradients and lack of capability for quantitative analysis. By contrast, development of microfluidic devices have been increasingly applied to cell migration studies owing to its ability to configure well-defined and stable chemical concentration gradients and the advantages in miniaturization, low reagent consumption and the potential for high-throughput experimentation[12-17]. Therefore, microfluidic devices offer a new experimental platform for quantitative cell migration and chemotaxis studies.
Most microfluidics-based cell migration and chemotaxis studies require complicated control instruments and specialized research facilities beside the microfluidic device, which is expensive and bulky. For example, to capture the cell migration images, a microscope and a digital camera are necessary. For chemotaxis experiments, checking the chemical gradient is a fundamental step before starting the experiment. Additional high power lamp is usually necessary for checking the gradient. As the external facilities are usually very expensive, it prevents many interested scientists or students to directly engage microfluidic cell migration research. Additionally, these systems are very impractical to use in a conventional clinical setting, which hampers its development for clinical applications.
To generate a stable gradient for the chemotaxis experiment, lots of microfluidic gradient generators have been developed. Those strategies can be roughly divided into two major classes, one is the flow-based device where molecules are mainly transported by the laminar flows [12]; the other is the free-diffusion based device where the molecules are mainly transported by the molecular diffusion without flows [18, 19]. Both types of devices are able to generate well defined gradients. The advantage of the flow-based chemotaxis device is the short gradient establishing time, stability and flexible gradient configurations. However this kind of devices usually requires external mechanical pumps to infuse the chemicals in a constant flow rate. This will increase the cost of the system and make the system inconvenient to set up. The advantage of the diffusion-based microfluidic device is that the cells are not subject to fluid flow induced shear stresses and less relies on external control systems. The disadvantage is that the gradient establishment time is long and less flexible to manipulate gradient profiles. Further efforts are needed to find a low-cost and easy strategy for rapid and stable gradient generation.
There are two main applications for the use of microfluidics in healthcare: POC testing and central laboratory diagnostics. Compared to the central laboratory diagnostics systems, the POC systems have lots of advantages; firstly it can be used in many places outside the laboratory, such as the patient's home, moving vehicles; secondly the time to result can be short; the cost of the test can be more affordable for patients. Because microfluidic devices are disposable, rapid in performing the test and require reduced amount of reagents, it is expect to find broad POC applications. A microfluidic system is usually composed by a disposable microfluidic chip and peripheral equipment (pumps, reader, etc.). In the specific cell migration studies, the cost of microfluidic chips can meet the requirement of POC testing. But the commercial peripheral equipments used to automatically capture and analyze the data are still expensive. For example, EZ-TAXIScan (ECI Inc., Japan) is a commercialized optical assay device for the quantitative measurement of cellular chemotaxis. This system, has a compact body but is expensive and requires special microfluidic chip, not allowing other customized chip designs.
The following references, incorporated herein by reference, relate generally to the present invention and are referred to throughout the current specification by number.    1. Keller, R., Cell migration during gastrulation. Curr Opin Cell Biol, 2005. 17(5): p. 533-41.    2. Matsubayashi, Y., et al., ERK activation propagates in epithelial cell sheets and regulates their migration during wound healing. Curr Biol, 2004. 14(8): p. 731-5.    3. McDougall, S., et al., Fibroblast migration and collagen deposition during dermal wound healing: mathematical modelling and clinical implications. Philos Transact A Math Phys Eng Sci, 2006. 364(1843): p. 1385-405.    4. Luster, A. D., R. Alon, and U. H. von Andrian, Immune cell migration in inflammation: present and future therapeutic targets. Nat Immunol, 2005. 6(12): p. 1182-90.    5. Friedl, P. and K. Wolf, Tumour-cell invasion and migration: diversity and escape mechanisms. Nat Rev Cancer, 2003. 3(5): p. 362-74.    6. Yamaguchi, H., J. Wyckoff, and J. Condeelis, Cell migration in tumors. Curr Opin Cell Biol, 2005. 17(5): p. 559-64.    7. BOYDEN, S., The chemotactic effect of mixtures of antibody and antigen on polymorphonuclear leucocytes. J Exp Med, 1962. 115: p. 453-66.    8. Zigmond, S., Ability of polymorphonuclear leukocytes to orient in gradients of chemotactic factors. J Cell Biol, 1977. 75(2 Pt 1): p. 606-16.    9. Lohof, A., et al., Asymmetric modulation of cytosolic cAMP activity induces growth cone turning. J. Neurosci., 1992. 12(4): p. 1253-1261.    10. Zicha, D., G. Dunn, and G. Jones, Analyzing Chemotaxis Using the Dunn Direct-Viewing Chamber. 1997. p. 449-457.    11. Nelson, R. D., P. G. Quie, and R. L. Simmons, Chemotaxis Under Agarose: A New and Simple Method for Measuring Chemotaxis and Spontaneous Migration of Human Polymorphonuclear Leukocytes and Monocytes. J Immunol, 1975. 115(6): p. 1650-1656.    12. Lin, F. and E. Butcher, T cell chemotaxis in a simple microfluidic device. Lab Chip, 2006. 6(11): p. 1462-9.    13. Saadi, W., et al., A parallel-gradient microfluidic chamber for quantitative analysis of breast cancer cell chemotaxis. Biomed Microdevices, 2006. 8(2): p. 109-18.    14. Saadi, W., et al., Generation of stable concentration gradients in 2D and 3D environments using a microfluidic ladder chamber. Biomedical Microdevices, 2007. 9(5): p. 627-635.    15. Ahmed, T., T. S. Shimizu, and R. Stocker, Bacterial Chemotaxis in Linear and Nonlinear Steady Microfluidic Gradients. Nano Letters, 2010. 10(9): p. 3379-3385.    16. Lin, F., Chapter 15. A microfluidics-based method for chemoattractant gradients. Methods Enzymol, 2009. 461: p. 333-47.    17. Kim, S., H. J. Kim, and N. L. Jeon, Biological applications of microfluidic gradient devices. Integrative Biology, 2010. 2(11-12): p. 584-603.    18. Abhyankar, V. V., et al., Characterization of a membrane-based gradient generator for use in cell-signaling studies. Lab on a Chip, 2006. 6(3): p. 389-393.    19. Si, G., et al., A parallel diffusion-based microfluidic device for bacterial chemotaxis analysis. Lab on a Chip, 2012. 12(7): p. 1389-1394.    20. Dertinger, S. K. W., et al., Generation of gradients having complex shapes using microfluidic networks. Analytical Chemistry, 2001. 73(6): p. 1240-1246.